This invention pertains to new and improved synthetic semi-permeable membranes or barriers and to processes for making these membranes.
The terms "membrane" and "semi-permeable" are on occasion used in various different manners. Because of this it is considered desirable to define the meanings attributed to these words as they are utilized in this specification in order to minimize the chances of confusion.
The term "membrane" as used herein is intended to designate a comparatively thin physical structure of such a character that certain substances such as water or other solvents can pass through it while others such as, for example, colloidal particles cannot pass through it. Because of this meaning of the word membrane it is considered that any membrane can be defined as or considered as a form of a barrier which will pass at least common solvents and in some cases comparatively small particles but which normally will retain or block the passage of particles or substances.
The term "semi-permeable" as used in this specification is intended to designate the ability to pass a solvent or solvent mixture but not to pass various dissolved or colloidal substances. The word "porous" indicates pore of any size or size range capable of retaining particles or molecules primarily because of the sizes of such particles or molecules.
Most commonly the term "porous membrane" is used to designate membranes which will pass a solvent, normally water, but which will not particles in excess of about 10 microns in diameter. On occasion porous membranes are referred to as microfiltration, ultrafiltration, membranes in accordance with the sizes of the particles which will or will not pass through them. A reverse osmosis membrane must have a porous layer and a nonporous, semi-permeable layer capable of passing a solvent by a solution mechanism. On occasion porous membranes are also referred to as microporous or as sterilizing membranes depending upon the pore sizes within such membranes. These pore sizes of course reasonably correspond to the sizes of particles or substances which can be passed through these membranes.
It is not believed that an understanding of the present invention requires a detailed understanding of the pore sizes of various different specific porous or semi-permeable membranes as are indicated in the preceding paragraph. It is, however, considered that the present invention requires an understanding of the fact that any such membrane may consist essentially of a single porous or semi-permeable layer or may consist of a plurality of individual separate layers. When a porous membrane consists of a plurality of layers normally the various individual layers will be employed so as to serve separate, different functions. Normally at least one of such individual layers will in and of itself be capable of being utilized as a single layer porous membrane.
This can be illustrated by referring to a known type of reverse osmosis membrane which, in its simplest form, consists of a synthetic, porous, somewhat flexible, thin film or membrane which serves as a physical support for an active layer of an essentially non-porous character. This active layer in this composite membrane is substantially inert and is capable of separating water from dissolved salts by dissolving water so that water can move from one surface of the active layer to the other. This composite structure falls within the definition of membrane indicated in the preceding discussion. The support film also comes within the scope of the definition of membrane indicated in the preceding.
In the particular composite reverse osmosis membrane described the porous support is utilized to provide a degree of physical reinforcement to the active layer so as to prevent rupture of the active layer and to provide passages which are suitable to convey liquid which has passed through the active layer so as to be separated from other materials away from the active layer. The support may in and of itself be utilized as a porous membrane or barrier inasmuch as the pores in the support are of such a physical dimension as to be capable of separating out various types of particles, such as, for example, colloidal particles when the support is used by itself in an appropriate structure.
Those familiar with the field of porous membranes, and in particular, the field of reverse osmosis membranes, will recognize that many different characteristics of such membranes are important from a utilitarian and commercial standpoint. An understanding of the present invention is also not considered to require a discussion as to all of such characteristics.
It is believed, however, that an understanding of the present invention requires a review of the fact that it is important that any porous membrane used be free or apparently free from discontinuities or other similar defects which might allow leakage through the membrane. It is also believed that an understanding of the present invention also requires a recognition of the fact that the speed with which a liquid can move through a porous membrane is quite important from a commercial standpoint. In general, the greater the ability of a porous membrane to pass a liquid the smaller the equipment capacity required to process a given volume of a suspension or solution so as to separate liquid from it.
In order to achieve relatively high flow rates for a liquid or liquid mixture through a semi-permeable or porous membrane it is desirable to form the membrane of a comparatively "stiff" or physically strong polymer or polymer system which will not rupture under normal conditions of use and which will not tend to compress significantly so as to close up or decrease the sizes of the pores in the membrane when subjected to working pressures. It is also desirable for the membrane to be of such a character that the liquid separated with it will wet the surfaces of the pores of the membrane. This latter is not only important in connection with the flow rates achieved, but is significant in another unrelated regard.
A common method of testing a membrane for a possible leak or rupture involves measuring the pressure required to force air through a membrane which has been wetted with water. This is a so-called "bubble point" test. The so-called bubble point increases with decreasing pore size in the membrane and is affected by the hydrophilic character of the membrane. A hydrophobic membrane or a membrane which has lost its ability to be wetted by water may exhibit the characteristics of a membrane having a ruptured surface even though, in fact, the membrane may be without any defect. As a consequence of this possibility of an erroneous indication of a defect, the wetability of the membrane is important so that the membrane will be accepted for many "critical" uses where any surface discontinuity would be undesirable.
The physical requirements necessary to achieve the strength characteristics of a desirable porous or semi-permeable membrane and other desirable characteristics are best achieved by forming a porous membrane as noted from a hydrophobic or essentially hydrophobic polymer or polymer system. When a membrane of this type is manufactured by sintering techniques the interiors of the pores of the membrane are clearly not hydrophilic. This is also true when a membrane is formed by evaperation of a solvent or solvent system or by mechanical fissuring of a sheet. When hydrophobic porous or semi-permeable membranes are manufactured by quenching a dope in a liquid bath, normally the resulting membranes will exhibit desirable hydropholic characteristics until they are dried. As a result of drying, these hydrophilic-like characteristics are lost from this type of membrane.
A number of ways have been proposed for imparting desirable hydrophilic characteristics to a hydrophobic or essentially hydrophobic membrane as indicated in the preceding discussion. Because of the inherent nature of the hydrophobic polymers or polymer systems these characteristics cannot be created or restored merely by immersing a membrane as noted in water.
It has been proposed to overcome this problem by oxidizing the interiors of the pores within such membranes with a strong oxidizing agent so as to cause such surfaces to become essentially hydrophilic in character. Such oxidation procedures have not been advantageous because of costs and because of difficulties in producing a uniform product under commercial conditions. It has also been proposed to impart desired hydrophilic characteristics to hydrophobic membranes by treating these membranes with a solution of one or more common surfactants. While this expedient will impart hydrophilic characteristics, to a hydrophobic membrane these characteristics will be lost in using or testing the membrane as a result of the wetting agent being leached from the membrane.